1. Field of the Invention
This invention relates to a single trip whipstock assembly, that is to say a whipstock assembly which can be run complete into a wellbore, set, and operated to mill a window in the casing of the wellbore and open up a lateral in the surrounding formation in a single trip.
2. The Prior Art
In the drilling of oil and gas wells it is sometimes necessary to form a branch extending off an existing bore. Such branches (generally known as "laterals") are in general formed by locating a tapering deflector device (known as a "whipstock") in the existing bore, and then using the whipstock to deflect a milling tool laterally of the axis of the existing bore to mill a window in the surrounding casing. Once the window has been milled, drilling of the surrounding formation can continue using the milling tool or the milling tool can be withdrawn from the well and replaced by an appropriate formation drilling assembly.
Heretofore, the initiation of a lateral using a whipstock system has necessitated a number of separate trips into the well. Typically, a packer is first run into the well and, using appropriate equipment, is located at the correct depth and orientation and is set. The packer string is then removed from the well, leaving the packer in position, and the whipstock, possibly in association with one or more mills, is run into the well to mate with the packer prior to commencement of the milling operation.
Various attempts have been made to increase the efficiency of the formation of a lateral using a whipstock system, and in particular U.S. Pat. No. 5,109,924 illustrates a window milling assembly to make the cut out and mill the window in one run, and U.S. Pat. No. 5,425,419 illustrates a system in which a deflector head at the top of the whipstock is used to locate the milling tool. All prior art systems, however, require some form of anchor or packer to locate the whipstock at the correct depth and orientation. Such anchors or packers must be run into the well on a separate trip from the whipstock proper necessitating at least two trips in any whipstock milling operation.